PROJECT SUMMARY/ABSTRACT Untreated opioid use disorders (OUDs) can have devastating consequences for people with serious mental illness (SMI). While the size of the population with unmet need is unknown, studies indicate that 87% of US adults with co-occurring disorders do not receive SUD treatment, and large numbers of individuals with mental illness are on chronic prescription opioids, a risk factor for heroin use and the development of an OUD. Untreated OUDs in the SMI are important because OUDs increase morbidity and mortality and are associated with higher healthcare and social costs, homelessness, and incarceration. Increasing access to OUD treatment and improving patient outcomes in the SMI will require addressing both the supply of and the demand for treatment. At the system and provider level, the availability of treatment needs to be increased. At the patient level, patient demand for treatment needs to increase, by identifying and addressing patient perceptions of need, desire and preferences for treatment. In this developmental R34, we propose to evaluate system, provider and patient-level facilitators and barriers, and then to use this information to develop an implementation strategy and toolkit to promote the use of medication assisted treatment (MAT)--the use of FDA-approved medications for OUDs in combination with behavioral therapies--for people with COD receiving public mental health treatment. The results of the R34 will prepare us for a future R01 study of the effectiveness of the implementation strategy and toolkit on MAT adoption, implementation and sustainability. We focus on MAT because of its demonstrated effectiveness and cost-savings, and specialty mental health because of the large role the mental health system plays in treating individuals with COD. Aims 1-3 seek to assess organizational capacity (at the system and provider level); organizational readiness (at the provider level); and perceived needs, attitudes and preferences (at the patient level). In Aim 4, we will use findings from Aims 1-3 to guide development of the implementation strategy and toolkit, using stakeholder input and a systematic process for strategy development. By implementation strategy we mean a group of implementation interventions that will address barriers at multiple levels. By toolkit, we mean the resources providers and clinics will need to execute implementation. To conduct the research, we will collaborate with the Los Angeles County Department of Mental Health, the largest mental health department in the United States. Despite serving nearly 20,000 individuals with COD, in 2016 only 37 prescriptions for MAT were written. Using a mixed methods approach, we will conduct interviews, focus groups, and surveys with patients and providers from 8 clinics serving an ethnically and geographically diverse population. Our study is a first step towards increasing access to OUD treatment for a vulnerable, costly and underserved population. Our approach is innovative because we consider barriers from the system, provider, and patient perspectives, and address both organizational supply and patient demand.